The present invention relates to a magnetic recording medium for ultra-high density recording with higher running durability. In particular, the invention relates to a floppy disk for ultra-high density recording with a ferromagnetic thin film and usable at high-speed rotation.
In recent years, with the purpose of recording a large mount of digital information, there are increasing demands n the development of a technique for recording at high recording density. Using a ferromagnetic metal thin film prepared by vacuum film forming method, i.e. sputtering method, vacuum evaporation method, etc., the magnetic recording medium such as a vacuum-deposited tape or a metal thin film type hard disk, etc. is prepared and used in practical application. In this type of magnetic recording medium, high magnetic energy can be easily obtained, and flat surface can be easily achieved by smoothening the surface of non-magnetic substrate. In this respect, it is characterized in that spacing loss is low and high electromagnetic transfer characteristics can be attained, and this is suitable for a method to manufacture a high-density recording material.
On the other hand, compared with the hard disk, a floppy disk type magnetic recording medium has higher impact resistance and can be produced at lower cost, and it is widely used in 2HD class products. Further, in recent years, a high-density magnetic recording medium represented by Zip (Iomega Inc.) based thin layer coating technique has been introduced to practical use. In the magnetic recording medium as described above, recording and reproduction are performed at high speed of about 3000 rpm, and high transfer rate closer to that of hard disk can be achieved. However, its recording density is still less than {fraction (1/10)} of that of the hard disk. This is attributable to the fact that a floppy disk type magnetic recording medium which has the magnetic layer produced by sputtering method as in case of the hard disk is not yet suitable for practical application.
There are various reasons for this. One of the reasons is that, in a floppy disk produced by the sputtering method, it is difficult to maintain high running durability and to attain high reliability for practical use.
In the floppy disk produced by the sputtering method, it is more difficult to maintain high durability than the floppy disk produced by the coating method. The reasons are that the magnetic layer of the floppy disk produced by the coating method comprises magnetic particles and inorganic particles, etc. enclosed by organic binder and the uppermost layer is the organic binder, that there are gaps between particles, and also that the magnetic layer contains a lubricant. As a result, the floppy disk of this type has high wear resistance, high shock resistance, and better lubricating property, while the floppy disk produced by the sputtering method has a magnetic layer comprising metal thin film which is more susceptible to wearing.
For this reason, there have been attempts to form a solid protective layer on the magnetic layer as in the case of hard disk for the purpose of improving the durability. Above all, it is known that a carbon protective layer has high durability. However, even when a carbon protective layer similar to the case of the hard disk are formed on the floppy disk, running durability can be improved but it is not possible yet to attain high reliability for practical use. In case of hard disk, when the number of revolutions of disk is increased, the head is floated up by floating force applied on the head, and it is used under the condition that the head and the hard disk are not in contact with each other. In case of floppy disk, even when the number of revolutions of the disk is increased, vibration of the disk (surface deviation) is high, and the head cannot be floated up in stable manner. Even at high-speed rotation, the head and the floppy disk come frequently into contact with and strike against each other.
For this reason, with the purpose of increasing the running durability and of maintaining high reliability for practical use, the floppy disk must have the following components and properties:
(1) Lubricating layer having high lubricating property;
(2) Protective layer with high wear resistance;
(3) Well-controlled surface roughness not to generate high frictional force even when head and disk are in contact with each other;
(4) Well-controlled surface hardness not to cause detachment or destruction even when head and disk come into contact with each other; and
(5) Low surface deviation to reduce frequency of contact.
In particular, unlike the case of the hard disk, which is basically operated on non-contact basis, it is necessary to design a magnetic recording medium by giving consideration on the contact between the head and the disk during running operation.
As the protective layer with high wear resistance, an amorphous carbon film containing carbon and hydrogen and called xe2x80x9cdiamond-like carbonxe2x80x9d (DLC) is most commonly used. It is used as the protective layer for hard disk or for video tape having metal thin film type magnetic layer and formed by vacuum deposition. Diamond-like carbon can be relatively easily produced. It is hard and has low frictional force and is very unlikely to cause seizure.
However, it has been found that, when the commonly used diamond-like carbon containing carbon and hydrogen is used as the protective layer for floppy disk, sufficient durability cannot be obtained. The reason for this is that, when diamond-like carbon is used as a protective layer, frictional force is gradually increased as the head and the disk are repeatedly brought into contact with each other. As a result, the diamond-like carbon may be destroyed or the magnetic layer may be destroyed. On the contrary, soft DLC cannot satisfy the requirements on wear resistance.
Further, regarding the lubricant, it has been not possible to improve the running durability of the floppy disk produced by the sputtering method to full extent.
As a ferromagnetic metal thin film type magnetic recording medium, which comprises a ferromagnetic metal thin film formed by vacuum film forming method and provided on a non-magnetic substrate as a magnetic layer and a protective layer provided on the magnetic layer, JP-B-52-18001 describes a thin film magnetic recording member, which comprises a thin film magnetic recording medium with an inorganic insulating amorphous material coated on it. Also, the conditions for providing a carbon protective layer on a magnetic recording medium are defined in each of JP-A-61-45412, JP-A-62-298923, JP-A-6-195695, etc. However, there has been no invention which directly defines the property of the protective film.
JP-B-5-56567 describes a magnetic recording medium, which has a carbon protective layer with a specific Vickers hardness formed on a ferromagnetic metal thin film. However, it is not possible to be accurately evaluate the property of thin film by Vickers hardness.
JP-A-6-349055 describes a magnetic recording medium by defining average roughness on central line of the carbon protective layer. Although it is defined according to the measurement results of micro-hardness, it is not valid enough as a method for evaluation.
It is an object of the present invention to provide a ferromagnetic metal thin film type magnetic recording medium for high density recording with higher running durability. In particular, the invention provides a floppy disk, which is suitable for high density recording at high-speed rotation.
The above object can be attained by the magnetic recording medium of the present invention, which comprises a magnetic layer containing a ferromagnetic metal thin film having micro-size projections on surface thereof and formed by vacuum film forming method and provided at least on one side of a nonmagnetic substrate, a carbon protective layer and a lubricant provided on said magnetic layer, a central axis of a diamond indenter of trilateral pyramid having a tip sharp angle of 90xc2x0 and a radius of curvature at tip of 35-50 nm being directed perpendicularly to the surface of the nonmagnetic substrate, pressure is applied on the carbon protective layer, and a load obtained when indentation depth of the indenter is 10-25 nm is divided by projection area of indenter contactor, and the quotient, i.e. micro-hardness, is within the range of 10-30 GPa.
Also, the present invention provides a magnetic recording medium as described above, wherein said nonmagnetic substrate is a flexible plastic substrate, and the magnetic recording medium is a disk-type magnetic recording medium.
Further, the present invention provides a magnetic recording medium as described above, wherein thickness of the carbon protective film is within the range of 2.5 to 30 nm.
Also, the present invention provides a magnetic recording medium as described above, wherein the carbon protective layer is formed using ethylene as raw material gas.
Further, the present invention provides a magnetic recording medium as described above, wherein the nonmagnetic substrate is aramide film or polyimide film.
Also, the present invention provides a magnetic recording medium as described above, wherein an undercoating film comprising polyimide resin or organopolysiloxane is formed on the nonmagnetic substrate.